This invention relates to a brushless tachometer generator.
A number of methods and devices, which rely upon closed loop automatic control systems, have been proposed and put into use for the control of various DC motors. In order to maintain a prescribed speed at a high level of precision it is necessary with all of these methods and devices to effect automatic control in such a manner that the speed of the DC motor is detected accurately and made to conform to a certain set value. It is conventional practice to employ only DC tachometer generators to detect the speed of the DC motor, in order to provide good linearity. On the other hand, because induction motors are simple in construction, durable and require little maintenance, particularly induction motors of the squirrel-cage type, methods have been developed in which closed loop automatic control systems are applied to such motors to control their speed. However, DC tachometer generators are employed even in these induction motor speed-control systems in order to detect the motor speed. These DC tachometer generators generally employ a brush and commutator system and require the expenditure of great amounts of labor for maintenance and inspection. In a speed control system for a squirrel-cage induction motor, an exorbitant amount of time is required just for the maintenance and inspection of the brush- and commutator-type DC tachometer generator itself. Accordingly, tachometer generators have recently been used in which a permanent magnet is adopted as a rotor and polyphase alternating currents are generated and then rectified by various methods using a contactless system, with the resulting DC output being utilized as a speed signal. For various reasons, one being the desire for a large output, the rotor is designed to have a width which is approximately equal to that of the stator so that, owing to the effects of coil end flux leakage, a variance occurs in the peak value of the output voltage generated in the coils. As a result, the tachometer generators of the type described cannot assure a high level of precision because of a ripple which exists in the DC output.
Another problem is associated with a Hall device which is employed at a portion of the set-up that detects a commutator switching operation. Specifically, with the conventional apparatus, the commutator section and generator section must be remote from each other so that the Hall device will not be affected by the flux leaking from the generator. The unfortunate result is that the length of the tachometer generator along its axis of rotation becomes quite large.